What is the value of an 1805 us quarter?

You should have your coin authenticated by a certified dealer or appraiser. If genuine, retail values as of 07/2012 are

Very worn - $530

Moderately worn - $1050

Slightly worn - $3000

Almost no wear - $6000 or more

An American quarter from 1805 is also known as a "draped bust" quarter for the engraving on the face. If uncirculated, these coins are worth thousands of dollars, but even worn 1805 quarters are worth a few hundred dollars due to their rarity and age.